1. Field of the Invention
The present invention relates to portable electronic devices such as audio players and digital cameras of the portable type, and more particularly to portable electronic devices having two power supply systems from an external power source and that from an internal power source.
2. Description of the Related art
Portable electronic devices such as audio players or digital cameras of the portable type conventionally have incorporated therein a dry battery or secondary battery serving as the power source to realize the portability of the device. The devices are adapted to be connected to external power sources such as commercial alternating power source, etc., for charging the secondary batteries.
In recent years, the USB (Universal Serial Bus) has attracted attention as a universal interface for connecting a plurality of peripheral devices in common to a host computer. The USB connector, which is compliant with the USB standard, comprises a pair of data terminals D+ and D−, a power source terminal, and a grounding terminal. It can provide high speed data communication using data terminals D+ and D−, and can be used for supplying power by way of the power source terminal. Accordingly, a method is studied that a portable electronic device is provided with a USB connector and the connector is, for example, connected to a personal computer to supply power to the device.
It is usual practice with the portable electronic device (compliant with the USB standard) having the USB connector to provide a main CPU for executing predetermined device operation processing for various operations of the device including reproduction of data, and a USB controller separate from the main CPU and adapted to execute predetermined data processing for carrying out data communication with the personal computer through the USB connector so as to ensure simplified processing.
When the USB compliant device, for example, downloads the compressed audio data from a personal computer to store the data on the memory card incorporated therein, and thereafter to read the data stored in the memory card to reproduce the data, the audio data is read out from the memory card with the data processing executed at about 128 Kbps, which is a relatively low speed. On the other hand, when the audio data is downloaded from the personal computer through the USB connector, the audio data which records sound for an hour is about 64 MB even if the data is compressed. If the transferring speed of 12 MbPSA, which is theoretical value of the USB, is drawn out to a maximum limit, it is necessary to take 40 seconds or more in transferring the data. Thus, because the data needs to be transferred as fast as possible, power source voltage of the IC for executing the data transferring should be set as high as possible.
The applicants have developed a portable electronic device wherein one chip IC conducts the control of write/read of the data to the memory card and the processing of data transfer (USB transfer) through a USB connector. With the IC provided with the portable electronic device, while the supply of the power source voltage greater than 2.7 V makes it possible to read the data from the memory card, the power source voltage greater than 3.0 V should be provided in order to realize high speed data transferring.
However, in the case where the USB connector is not connected to the personal computer to reproduce the data using internal power source including a dry battery, a secondary battery and the like, the internal power source is discharged with high voltage of greater than 3.0 V as described above. This entails the problem of shortening the life of the internal power source. Although the power source voltage is reduced to 2.7 V to prolong the life of the internal power source, the USB transferring speed is declined.